Light vehicle range discriminator

ABSTRACT

In combination, intrusion detection apparatus giving a signal when a light vehicle approaches a geophone, and a heavy vehicle discriminator disabling the detection apparatus when the approaching vehicle is a heavy one.

The Government has rights in this invention pursuant to ContractF0865-79-C-0178, awarded by the Department of the Air Force.

TECHNICAL FIELD

This invention relates to the field of armaments, and particularly togeophone apparatus for use with land mines.

BACKGROUND OF THE INVENTION

Occasion sometimes arises to interdict the movement of vehicles in anoutdoor area. This may be done by distributing mines, throughout thearea, which function upon firing to disable a vehicle within aneffective range from the mine. Mines are accordingly equipped withgeophone apparatus for detecting the approach of targets within therange thereof.

It is desirable that such mines be not readily perceptible, and thusavoidable, by approaching vehicles: passive vehicle detectingarrangements are accordingly preferred. In this respect, seismic devicessuch as geophones, supported on or buried in the ground and responsiveto vibrations in the earth caused by the vehicles, have been found quitesatisfactory, as is taught in U.S. Pat. No. 4,468,763. A difficultyarises, however, since in such arrangements the seismic signal due to alight wheeled vehicle near to the geophone is not readilydistinguishable from the seismic signal due to a heavy tracked vehiclemore remote from the geophone. Some land mines are also known to beeffective against light vehicles, such as jeeps, but to be relativelyineffective against heavier vehicles, such as armored tanks. It isinefficient to cause a mine to fire when its target is too remote forthe mine to be effective, or when the target is one against which themine is ineffective, since mines are expensive, high-technology devices.

Apparatus has been developed for receiving seismic vibrations from anapproaching vehicle, distinguishing whether the seismic source is aheavy vehicle or a light vehicle, and giving a control signal if thesource is a heavy vehicle. One arrangement for accomplishing this istaught in my co-pending patent application Ser. No. 753,733, filed July10, 1985 and assigned to the assignee of the present application.

SUMMARY OF THE INVENTION

The present invention comprises seismically responsive means fordetecting the presence of approaching vehicles, means distinguishingseismic signals due to heavy vehicles, means giving a control signalwhen a light vehicle is within a predetermined range, and meansinhibiting the control signal if the seismic signal source is determinedto be a heavy vehicle.

Various advantages and features of novelty which characterize theinvention are pointed out with particularity in the claims annexedhereto and forming a part hereof. However, for a better understanding ofthe invention, its advantages, and objects attained by its use,reference should be had to the drawing which forms a further parthereof, and to the accompanying descriptive matter, in which there isillustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing, in which like reference numerals identify correspondingparts throughout the several views,

FIG. 1 is a block diagram of the combination of elements making up theinvention, and

FIG. 2 is a more detailed showing of the apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, an intrusion detecting device 10 is connectedat 11 to a geophone 12, and gives an output 13 when an intruding vehicleis detected by the geophone. Also connected to geophone 12, at 14, is aheavy vehicle discriminator 15, which gives output 16 when a heavyvehicle is detected. Outputs 13 and 16 are connected to a logical AND 18so that a control signal is supplied at 19 when a light vehicle atproper range is detected, but is inhibited if the seismic source is aheavy vehicle. Signal 19 may be used to control the firing of a minewith which the geophone is associated, or for any other desired purpose.

Turning now to FIG. 2, one form of an intrusion detector 10 is shown tocomprise a low frequency channel 111 including a preamplifier 112connected to 11 to geophone 12 and supplying a signal through a bandpassfilter 114 having a pass band of about 1.5 to 8.5 Hertz, an isolationamplifier 116, and a full wave rectifier 118 as an input 119 to ashort-term averager 122 having a time constant of about 4 seconds and aninput 123 to a long-term averager 124, having a time constant of about12 seconds. The averager outputs 126 and 128 respectively are suppliedto a comparator 120, together with the output 130 of an offset device132, and comparator 120 gives signal 13 when a vehicle is within apredetermined range of the geophone.

FIG. 2 also shows that in discriminator 15 geophone signal 14 issupplied to a wide band preamplifier 214, which energizes bandpassfilters 216 and 218 with signals 220 and 222. Filter 216 is a part of ahigh frequency channel 223, and preferably has a pass band of about 20Hertz to 150 Hertz. Filter 218 is part off a low frequency channel 224,and preferably has a pass band of about 1.5 Hertz to 8.5 Hertz. Channel224 has a considerably higher gain than channel 111 of detector 10.

The output 225 from filter 216 is fed to a full wave rectifier 226 whichsupplies inputs 228 and 230 respectively to a first low pass filter 232,acting as a short-term averager, and a second low pass filter 234,acting as a long-term averager. Filter 232 preferably has a timeconstant of about 8 seconds, and filter 234 preferably has a timeconstant of about 16 seconds. The outputs 236 and 238 respectively offilter 232 and 234 are supplied to a comparator 240, together with theoutput 242 of an offset device 244.

The output 246 from filter 218 is fed to a full wave rectifier 248 whichsupplies inputs 250 and 252 respectively to a first low pass filter 254,acting as a short-term averager, and a second low pass filter 256,acting as a long-term averager. Filter 254 preferably has a timeconstant of about 4 seconds, and filter 256 preferably has a timeconstant of about 12 seconds. The outputs 258 and 260 respectively offilters 254 and 256 are supplied to a comparator 262, together with theoutput 264 of an offset device 266.

The output 268 of comparator 240 is fed through an inverter 270 tocomprise a first input 272 to an OR circuit 274, having a second input276 from an oscillator 278 and a feedback input 280 comprising theoutput of a counter 282 energized at 284 from OR circuit 274.

The output of counter 282 is also supplied at 286 to an inverter 288which supplies a first input 290 to an AND circuit 292. A second input294 to AND circuit 292 is supplied by comparator 262, and the output 296of AND 292 is supplied through a latch 298 and an inverter 300 toconstitute input 16 to AND circuit 18.

In computer terminology the outputs of counter 282, AND circuit 292,latch 298, and comparators 120, 240, and 262 are normally LOW, and thoseof inverters 288 and 300, OR circuit 274, and inverter 270 are HIGH.

OPERATION

A moving heavy vehicle acts as a source of seismic vibrations of twotypes, identified respectively as Rayleigh waves and pressure waves. TheRayleigh waves occur in a high frequency band and a low frequency band,while the pressure waves occur only in a low frequency band, which isthe same however as that of the low frequency Rayleigh waves. Pressurewaves are more subject to attenuation than are Rayleigh waves, so thatfor an approaching heavy vehicle the Rayleigh waves are detected thesooner.

A moving light vehicle acts as a source of seismic vibrations of thesame types, except that there is no significant low frequency band ofRayleigh waves. The amplitudes of the light vehicle signals are alsosmaller, so that they are detected after a greater delay than the heavyvehicle signals.

When a geophone is coupled to the ground, there is always a seismicsignal at a background level. When this is rectified and sent toaveraging circuits such as 122 and 124, the outputs of these circuitscontinue generally at constant levels. By suitably selecting thecomponents of offset apparatus such as 132, the inputs to a comparatorsuch as 120 may be so set that in the absence of any signal from anapproaching vehicle, the long-term average 128 with offset 130 is alwaysgreater than the short-term average 126, so that comparator 120 producesno output 13.

In the same manner, offset devices 244 and 266 can be set so that in theabsence of any signal from an approaching vehicle, long-term averages238 and 260 with offsets 242 and 264 are always greater than short-termaverages 236 and 258, and comparators 240 and 262 provide no outputs.

When any vehicle approaches a geophone, the seismic signals in generalincrease as the vehicle moves closer. If a light vehicle approaches thegeophone, the initial low frequency seismic signal consists of pressurewaves. If a heavy vehicle approaches the geophone, the initial lowfrequency seismic signal consists of Rayleigh waves. Because of lessersoil attenuation, low frequency Rayleigh waves travel much further thanlow frequency pressure waves.

The low frequency seismic component to channel 111, whether Rayleigh orpressure waves, causes the signal produced by rectifier 118 to increase,so that outputs 126 and 128 both increase. However, because of thedifference in time constants in circuits 122 and 124, the output of theformer can increase more rapidly than the output of the latter until therelation of the signals to comparator 120 reverses, and the comparatornow gives an output 13.

The distance, from the geophone to the vehicle, at which output 13 isgiven may be adjusted by offset device 132, which is set so that thedistance is small enough for a light vehicle approaching the mine to beincapacitated. If the approaching vehicle is a heavy one, the initialseismic input contains low Rayleigh waves, and because of their lesserattenuation, the output 13 from comparator 120 occurs at an earliertime, when the heavy vehicle is far from the geophone which rendersineffective mine operation. To prevent this, output 13 is supplied tothe mine through AND circuit 18, which does not give its output 19 inresponse thereto unless discriminator signal 16 is also present,identifying the approaching vehicle as a light one, as will now bedescribed.

If the approaching vehicle is a heavy one, the low frequency Rayleighwaves act in channel 224 to cause comparator 262 to go HIGH at a timet₁, and the high frequency Rayleigh waves act in channel 223 to causecomparator 240 to go HIGH at a time t₂. It has been determined that, forheavy vehicles, time t₁ may occur before or after time t₂, but that timet₁ always occurs before a time t₃ which is later than t₂ by about 4.3seconds.

If t₁ precedes t₂, the output 294 of comparator 262 can pass through AND292 to comprise a HIGH output at 296, which passes through and is heldin latch 298, disabling AND 18 through inverter 300.

If t₂ precedes t₁, the output of comparator 240 goes HIGH, that ofinverter 270 goes LOW, and pulses from oscillator 278 may now passthrough OR 274 to counter 282. After a predetermined count is reached,corresponding to 4.3 seconds, counter outputs 280 and 286 go HIGH, theformer interrupting further counting and the latter acting throughinverter 288 to disable AND 292, and thus prevent any later operation ofcomparator 262 from affecting latch 298.

An approaching light vehicle will not act through discriminator 15 todisable AND 18 because for light vehicles comparator 262 does not giveits output 294 until after the 4.3 second delay in counter 282 hasdisabled AND 292.

From the above it will be evident that channels 223 and 224 cooperate tosupply signals which actuate latch 298 at a value which disables AND 18if an approaching vehicle is a heavy one. Channel 111 supplies a signalat 13 to AND 18 whether the vehicle is heavy or light.

Since channels 224 and 111 have much the same characteristics, it isdesired that channel 224 operate sooner than channel 111, which can beaccomplished by giving it a higher gain.

From the above it will be evident that the invention comprises intrusiondetetion apparatus for giving a signal when a light vehicle approaches ageophone, together with apparatus for disabling the detection apparatuswhen the approaching vehicle is a heavy one.

Numerous characteristics and advantages of the invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, and the novel features thereofare pointed out in the appended claims. The disclosure, however, isillustrative only, and changes may be made in detail, especially inmatters of shape, size, and arrangement of parts, within the principleof the invention, to the full extent indicated by the broad generalmeaning of the terms in which the appended claims are expressed.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:
 1. Apparatus for detecting andsignaling the intrusion (with respect to a point on the earth) of alight vehicle including means to prevent a signaling of a heavy vehicleintruding upon said point, said apparatus comprising in combination:(a)single geophone means mounted and adapted to respond to vibrations inthe ground and to provide an output indicative of vibrations sensedthereby; (b) first means including high-pass means connected to receivethe output of said geophone means, said first means further includingmeans for providing an output when the long-term average of highfrequency vibrations reaches a predetermined relation to the short-termaverage thereof; (c) second means including low pass means connected toreceive the output of said geophone means, said second means furtherincluding means for providing an output when the long-term average oflow-frequency vibrations reaches a pedetermined relation to theshort-term average thereof; (d) an oscillator means having an output forproviding pulses; (e) OR circuit means having three inputs and an outputmeans; (f) counter means having an input and an output; (g) first ANDcircuit means having a pair of inputs and an output; (h) latch meanshaving an input and an output; (i) means including inverter means forconnecting the output of said first means to one of said inputs of saidOR circuit means, said other inputs of said OR circuit means beingconnected to receive: (1) the output of said oscillator means, and (2)an output from said counter means, and said OR circuit output meansbeing connected to the input of said counter means; (j) means includinginverter means for connecting the output of said counter means to one ofthe input means of said first AND circuit means; (k) means connectingthe output of said second means to the other of the input means of saidfirst AND circuit means; (l) means connecting the output of said firstAND circuit means to the input of said latch means; (m) second ANDcircuit means having a pair of inputs and an output; (n) means includinginverter means connecting the output of said latch means to one of theinputs of said second AND circuit means; (o) third means including lowpass means connected to receive the output of said geophone means, saidthird means further including means for providing an output when thelong-term average of low-frequency vibrations reaches a predeterminedrelation to the short-term average thereof; and (p) means connecting theoutput of said third means to the other input of said second AND circuitmeans whereby said apparatus functions as follows:(1) upon the approachof a heavy vehicle toward said point said first and second means signalsare connected as aforesaid to thereby cause said latch means to operateto thereby signal the intrusion of a heavy vehicle, the operation ofsaid latch means in turn inhibiting said second AND gate from signal ina vehicle at the output thereof, (2) upon the approach of a lightvehicle toward said point, said first and second means functioncoactively so as to inhibit the operation of said latch means, and saidthird means functions to produce an output signal which is applied tosaid other input of said second AND circuit means to thereby produce asignal at the output thereof indicative of the presence of a lightvehicle.
 2. Apparatus of claim 1 further characterized by the output ofsaid second means occurring prior to the production of an output by saidthird means.
 3. Apparatus of claim 2 further characterized by said firstmeans having a band pass of between 20 and 150 Hertz.
 4. Apparatus ofclaim 2 further characterized by said second and third means having aband pass of between 1.5 and 8.4 Hertz.
 5. Apparatus of claims 1 or 2characterized by said first means having a band pass of between 20 and150 Hertz and said second and third means having a band pass of between1.5 and 8.5 Hertz.